The more and more serious requirements of technique led to the manufacture of structural steels with mechanical features which can be defined as medium-high strength features. Among said steels may be mentioned e.g. the types ASA 60, ASTM A517F, ASTM A5L7B, AISI 4130, ASTM A285, ASTM A537B and so on, whose tensile strength varies, according to the kind of steel, from about 35 kgs/mm2 to about 90 kgs/mm2. These steels on one side allow to achieve several advantages like lower weights, lower thicknesses and/or larger sizes in the structures made therefrom, but on the other side have disadvantages, among which the one referring closer the problem faced by the invention is that said steels are sensitive to the surrounding conditions, particularly as for the tendency to be subject to alterations favouring breaks by brittleness. To this case, one of the most dangerous agent is the class of sulphides present in most forms of pollution and further in most important products, like for instance petroleum; in fact, said sulphides favour strongly the development of hydrogen, and thus are to be considered as one of the main reasons for strains for short breaks in most of the materials with high mechanical characteristics.
That problem favoured researches throughout the world in consequence of which many compositions of carbon steel were obtained, without alloy elements other than carbon or with only a small percentage of such alloy elements, which are in position to oppose efficiently the phenomenon of sulphidic tensiocorrosion.
Among the numerous technical publications, may be mentioned U.S. Pat. No. 3,600,161 by Nippon Steel, the report by H. Kihara at the seventh World Petroleum Congress`, Proceeding, vol. 5(Mexico) 1967 pages 235-260 and the publication by R. W. Saehle et al `Stress Corrosion cracking and hydrogen embrittlement of iron-base alloys` St. Etienne Preprint n.Fl, June 12-14, 1973.
However, said metallurgical solutions do not still offer sufficient strength guarantees when welded structural elements are in contact with the sulphidic environment, owing to their high tendency to hydrogen cracking in the heat affected area, or when are provided cathodic protections which naturally increase the amount of hydrogen available for the embrittlement phenomena. These features in the use of high strength steels are e.g. pointed out in the publication `Studies on sulphide corrosion cracking of high strength steels` of 1963 by the Welding Association of Japan, and in an article by H. Kushiwaji and K. Shimoki in Japan Soc. for Safety Engineering, 5,314 (1966).
An article by J. F. Bates in `Materials Protection`, January 1969, pages 33-40 that phenomenon is studied with tests. In a sulphidic environment, most of the test elements without any protection have a very high percentage of cracking by hydrogen embrittlement in extremely short periods of time, which in the best cases do not reach one year.
Therefore, the situation of the use of structural steels with high mechanical characteristics in places polluted by sulphides, or anyhow where is available hydrogen in the elementary phase, is quite difficult, particularly as for the welded structures of transport and stocking tanks, pipes for crude oil, and so on.
Notwithstanding the great interest of the industry in the improvement of the resistance to the hydrogen embrittlement of welded joints, only a few suggestions suitable to oppose said phenomenon have been advanced.
As far as we know, there are two main methods for the protection against hydrogen embrittlement (besides obviously the embodiment of new compositions of steel) one consisting of subjecting the welded joints to thermal extension treatments, and the other of protecting the welded joints with a coating of e.g. varnishes or resin layers.
As it may be easily understood, both said methods, though efficient in absolute, have remarkable limits. As a matter of fact, the former cannot be used with very large structures, and the latter is obviously efficient for a limited period of time, after which a new coating must be applied.